It is well known that tougheners such as grafted rubbers or ionic polymers can be employed to improve the toughness of polyamides; see for example U.S. Pat. Nos. 4,174,358 and 3,845,163. It is also well known that plasticizers can be incorporated into polyamide blends to decrease their stiffness. N-butylbenzenesulfonamide (hereinafter BBSA) is an example of a well-known, effective plasticizer for polyamides; see, for example, Kohan, M. I. Ed. Nylon Plastics Handbook, Hanser: Munich, 1995; p. 365. Plasticized, toughened polyamide compositions such as ZYTEL® 350PHS2 NC010, manufactured by E.I. du Pont de Nemours, have been available for a number of years. Such resins are useful, for example, as jacketing for cables. However, while pipes and tubing can be made from such materials, it would be desirable to have available resins for making polyamide-containing pipes and tubing that possessed a lower flexural modulus, as such pipes and tubing would have great flexibility.
One method for lowering the flexural modulus would be to introduce higher levels of plasticizer to the polyamide composition. Thermoplastic polyamides are solids that are most efficiently blended on a molecular level with other materials by melting the polyamide in a suitable melt-processing apparatus such as an extruder and adding the additional materials to the molten polyamide and thoroughly mixing the resulting blend. When the blend is allowed to cool, the additional materials will be uniformly dispersed throughout the polyamide. As a result of high temperatures required for the melt-processing of polyamides, this method is not effective when large amounts of a volatile material must be added to the polyamide, as a significant portion of the volatile materials will be lost through the atmospheric pressure or vacuum vent port of the extruder. This requires that large excesses of the volatile material be added to the extruder to compensate for these losses, increasing the expense and complexity of the process.
Plasticized polyamide compositions are typically prepared by compounding the polyamide and other desired ingredients with the plasticizer in an extruder. However, because polyamide plasticizers are typically volatile relative to polyamides at the temperatures required to compound polyamides, it is difficult to incorporate large amounts of plasticizer using this method. For example, polyamide 6,12, a relatively low-melting polyamide, has a melting point of about 214° C. and BBSA has a boiling point of 340° C. Since it is necessary to compound polymers at temperatures well above their melting point, polyamide 6,12 is compounded at temperatures at which BBSA has a significant vapor pressure, and a significant portion of the BBSA introduced to the extruder would be lost through the vent port during compounding, making it impossible or very inefficient to achieve high loadings of BBSA. Other polyamides having higher melting points are compounded at even higher temperatures and will experience correspondingly higher losses of plasticizer. Plasticizers may be incorporated into polyamides during the polymerization process, but it is not practical or in some cases possible to incorporate additives such as tougheners or fibers or fillers during polymerization.
Higher molecular weight, less volatile plasticizers are available but they are not as efficient as BBSA, requiring even higher additive amounts. This negatively impacts other properties, such as tensile strength. Additionally, it is often desirable that plasticized polyamide resin compositions have a polyamide component with a high average molecular weight, and hence high melt and solution viscosity. Such compositions are often used for extrusion processes that form pipes, tubing, sheeting, etc where a high melt viscosity is desirable. When high molecular weight polyamides are passed through an extruder or otherwise melt-processed to incorporate other desired ingredients, however, these polyamides often are reduced in molecular weight, particularly when moisture is present in the polyamide, other ingredients, or extruder or other melt-processing equipment. Thus it is desirable to have a process that allows for the preparation of highly plasticized polyamides with a high average molecular that also contain additives. Such a process has been heretofore unknown and though they have long been desired, highly plasticized, high average molecular weight polyamides containing additives such as tougheners, reinforcing agents, and fillers are commercially unavailable, due to the difficulties that have been encountered when attempting to add both such additives and plasticizers to polyamides while efficiently using conventional melt-processing methods. These difficulties have been particularly acute when it is also necessary to maintain the molecular weight of high average molecular weight polyamides during the incorporation of plasticizer and additives.
It is an object of the present invention to provide an efficient process for producing a highly plasticized polyamide composition containing additives. It is a feature of the present invention to produce highly plasticized polyamide compositions containing additives such as tougheners and fillers. It is an advantage of the present invention to produce highly plasticized polyamide compositions containing additives wherein the polyamide has a high average molecular weight.